Heart disease resulting from pathological cardiac remodeling and hypertrophy is a leading cause of morbidity and mortality in the western world. Protease activated receptors (PARs) are expressed in cardiomyocytes and cardiac fibroblasts. However, their role in cardiac remodeling has not been studied in vivo. Importantly, we demonstrated that PAR-1-/- mice have reduced cardiac remodeling and left ventricle dilation at 2 weeks compared with wild type littermates in a cardiac ischemia-reperfusion (I/R) injury model. Furthermore, we showed that cardiomyocyte-specific overexpression of either PAR-1 or PAR-2 induced cardiac hypertrophy in mice. In addition, we found that PAR-1 and PAR-2 expression was increased in hearts from humans and mice with cardiac hypertrophy. Finally, in vitro studies showed that the activation of PAR-1 or PAR-2 on cardiomyocytes induced hypertrophy. Based on these results, we hypothesize that both PAR-1 and PAR-2 on cardiomyocytes and cardiac fibroblasts contribute to cardiac remodeling after I/R injury. The proposal has two Specific Aims. Specific Aim 1 will determine the role of PAR-1 and PAR-2 in cardiac remodeling after I/R injury. For these experiments, we will use both PAR-1-/- and PAR-2-/- mice, as well as PAR-1 and PAR-2 specific inhibitors. PAR-1flox/flox mice will be generated and crossed with mice expressing the Cre recombinase in either cardiomyocytes or cardiac fibroblasts. These mice will be used to determine the relative contribution of PAR-1 expressed on cardiomyocytes compared with cardiac fibroblasts to cardiac remodeling. Specific Aim 2 will evaluate the role of the MEK5-ERK5, MEK1-ERK1/2 and calcineurin signaling pathways in PAR-1- and PAR-2-dependent hypertrophy of cultured cardiomyocytes by inhibiting these pathways with pharmacologic inhibitors and dominant negative mutants. In addition, we will characterize the different signaling pathways that mediate hypertrophy in the hearts of mice overexpressing either PAR-1 or PAR-2 on cardiomyocytes. These studies will elucidate the role of PAR-1 and PAR-2 in cardiac remodeling after I/R injury and will determine the mechanism by which PAR-1 or PAR-2 activation induces cardiac hypertrophy. The results may lead to the development of PAR-1 and PAR-2 inhibitors that may be used as novel therapies for the treatment of pathological cardiac remodeling and hypertrophy in humans.